The effect of exogenous progesterone (P) on the corpus luteum function (in terms of the secretion of P and 20-α-dihydroprogesterone (DHP), on the secretion of prolactin (Prl) and on the pituitary responsiveness to LHRH was studied in pseudopregnant (PSP) rats kept in alternating and constant lighting conditions (LD-PSP and LL-PSP rats, respectively). Rats were rendered pseudopregnant by appropriately timed stimulation of the cervix uteri (LL rats first received an ovulatory dose of hCG). LH responses were induced by constant rate infusion of LHRH (104 ng/hfor 21 h). P was delivered by subcutaneously inserted Silastic implants; control rats received sham implants. In both LD- and LL-PSP rats the plasma P and DHP levels were high on day 8 of PSP. On day 12, however, the plasma P levels had fallen but the DHP levels had risen, demonstrating that between days 8 and 12 functional luteolysis had occurred and that neither the production of P and DHP, nor the timing of luteolysis are under the control of the lighting conditions. On day 12 of PSP the pituitary responsiveness to LHRH was much higher than on day 8. Moreover, on days 8/9 of PSP peaks of Prl were seen in all rats, but on days 11/12 such peaks were largely absent. In LD-PSP rats ‘nocturnal’ Prl peaks were seen on days 8/9 in all 9 experimental animals, but ‘diurnal’ peaks were seen in only 4 of these animals. Also, the diurnal peaks were on average much lower than the nocturnal peaks. In LL-PSP rats we saw on days 8/9 over 24 h 2–3 irregularly timed peaks of Prl, which were not as high as the nocturnal peaks but higher than the diurnal peaks of LD-PSP rats. After implantation of two Silastic P implants into LD- and LL-PSP rats on day 6 of PSP, i.e. before functional luteolysis, the peaks of Prl and a low pituitary LHRH responsiveness were still present on day 12. Also, on day 12 the plasma concentrations of DHP were not increased in P-implanted rats. P implants inserted on day 11 (i.e. after functional luteolysis) did not prevent cessation of the appearance of Prl peaks and the rise of DHP secretion in both LD- and LL-PSP rats. Also, in LL-PSP rats which were on day 6 of PSP both ovariectomized and implanted with P, peaks of Prl were still observed on days 11/12, but in animals with sham implants peaks were absent. It thus appears that P is the corpus luteum factor which provides a permissive environment for the neural signal which causes the secretion of peaks of Prl. It is concluded that (1) during PSP and until functional luteolysis the relatively high P levels play a role in the maintenance of the secretion of peaks of Prl, and that (2) maintenance of the corpus luteum function by exogenous P (and thereby the maintenance of the state of low LHRH responsiveness) is due to P postponement of